Common sense of electrical safety - part two

- Nov 07, 2017 -

4. Lightning accident

Lightning is atmospheric electricity and lightning strikes are the result of electrical energy in the atmosphere. Lightning strikes are natural disasters that can damage people and livestock in addition to destroying equipment and facilities, as well as causing fire and explosions. Thunderstorms should be thunderstorm lightning occurred in the summer, is the phenomenon of natural discharge developed from the Cumulonimbus. There are many reasons why rain clouds start to electrify, most of them think that the clouds in the clouds and ice crystals or graupel particles make the temperature below 0 ℃ cloud droplets in the above collision and freeze it, and in the event of freezing the surface fly debris cause. When there is a difference between the two ice spikes, the hot hydrogen ion diffuses faster and negatively than the hydroxide ions, while the cold one is positively charged. Once the ice crystal breaks, the positive and negative ions will separate into two small residues. On the other hand, when the droplets drop on the surface of the particle, the inner surface of the outer shell of the ice shell is negatively charged. When the shell is broken, the broken shell fragments are positively charged and the surface of the particle is negatively charged. In the upper part of the cloud, the graupel particles accumulate in the clouds at the bottom due to their heaviness. When the potential difference reaches a few hundred meters and a few thousand volts, there is a thunderstorm of swimming, which is lightning. The discharge between the clouds and clouds, although there is a lot of noise and intense lightning, little harm to people, only the clouds on the earth discharge will make buildings, electrical equipment or people and livestock and other damage and casualties, the destructive effect of the following Three causes:

     (A) direct lightning: thunderclouds directly to the discharge of ground objects, lightning strike time is very short, only a few tenths of a few seconds, but a very large current through, up to 100∽200 kA Air temperature suddenly rose to 1 ∽ 20,000 degrees Celsius, resulting in a strong shock wave, resulting in damage to housing, human and animal injuries. When the lightning current through a resistive or inductive object, can produce a great voltage drop and induced voltage, damage insulation, sparks, equipment damage, and even cause burning, explosion, the harm further expanded.

     (B) induction discharge: lightning is caused by the nearby electromagnetic effect of the results can be classified into two types of electrostatic induction and electromagnetic induction:

     Electrostatic induction is due to thunderclouds over the building, the building will be induced by the thundercloud charged charge opposite charge, Thundercloud began to discharge on the ground, the charge in the discharge path of the charge quickly and quickly, but the top of the building charge Not immediately dispersed into the ground, they form a high potential, resulting in the building within the wires, metal equipment, metal pipe discharge, causing fire, explosion and personal accident. Electromagnetic induction when the lightning current through the metal body into the ground, the formation of a strong magnetic field, the nearby metal conductors can be induced to a high potential, causing the spark gap in the conductor loop.

     (C) the introduction of high potential by the overhead lines: overhead lines in the direct lightning strike or in the vicinity of the lightning induced over-voltage, if not managed to make a lot of electric flow in the road to the ground, it will be introduced along the overhead lines inside the house, causing damage to housing or electrical Equipment insulation breakdown phenomenon.

5. Circuit failure

  Circuit failures are caused by the loss of control over the transfer, distribution and conversion of electrical energy. Failure of electrical wiring or electrical equipment may affect personal safety and abnormal power outages may also affect personal safety. These are circuit faults, but from a safety system perspective , The same should be noted that these unsafe conditions may cause accidents. Second, the physiological effects of current on the human body

    The effect of current on the human body refers to the harmful effects on the human body caused by the current passing through the human body. For example, current passing through the human body can cause acupuncture, oppression, fights, spasms, pain and even blood pressure, coma, arrhythmia, Fibrillation and other symptoms. Current through the human body, the severity of human injury and the body through the current size, duration, ways, types and human condition and other factors, especially with the current size and the power of time has a very close relationship.

    (1) the size of the current through the body of different current, causing the body's physiological response is also different. For the power frequency current, according to the size of the current through the human body and the different reactions presented by the human body, the current can be divided into sensing current, current and lethal current.

    ① sense of current is the minimum current that causes the human feeling. The initial feeling of electric current is a slight shaking and slight tingling. Experience has shown that the general adult male's perception of current is about 1.1mA, adult women is about 0.7mA

    ② get rid of the current refers to the body after the electric shock to get rid of the maximum current. The average male adult's current shedding current of 16mA, adult women is about 10.5mA, children out of the current than adults to be small. It should be noted that the ability to shed current is diminished as the duration of the shock is prolonged. This means that once the electric shock can not get rid of the power, the consequences will be more serious.

    ③ fatal current refers to the minimum current in a short period of time endanger human life. Electrical shock is caused by current-induced ventricular fibrillation. Therefore, the current caused by ventricular fibrillation is a fatal current. 100mA is a lethal current.

    (2) current duration The longer the current through the human body, the greater the risk of electric shock damage. Each human heart contraction and expansion of a gap of about 0.1s, 0.1s of the gap period of the current-sensitive, the longer the power, it is bound to coincide with the heart of the most sensitive gap caused by electric shock; the longer the power supply, the body resistance Due to stress sweating and other factors to reduce resistance, leading to further increase in current through the body, can cause electric shock.

    (3) the current through the human body current through the heart will cause ventricular fibrillation or heart beat, resulting in interruption of blood circulation, leading to death. Current through the central nervous system or related parts can cause death. Electric current through the spinal cord can cause paraplegia. Generally from the left hand to the chest is the most dangerous way; from hand to foot, from left to right are very dangerous current path; from foot to foot path although less injury, but after the fall, can cause current Through the serious condition of the whole body.

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